When coated paper products are folded, there exists a tendency for cracks to develop at the fold. This cracking can be referred to as flex cracking or cracking at the fold. Paper products that are often subjected to folding are often found in magazines, books, bags, and boxes. Flex cracking or cracking at the fold can be either cosmetic or may lead to a complete paper failure at the location of the fold. A cosmetic cracking can be characterized by a small disruption of the coating layer at the location of the fold, making the fold unappealing, especially if the fold is in a heavily inked area.
Factors that effect cracking at the fold include changes in humidity as a result of seasonal changes and the chemistry of the paper substrate, the coating, the ink, and the techniques for processing and manufacture. Cracking at the fold may become more noticeable if heavy colors are used at fold areas.
Several techniques are available for reducing cracking at the fold. One technique involves selecting a latex for the coating layer. Another technique is to increase the moisture in the coated paper. Increasing the moisture level can lead to other coating defects, such as, blister. These techniques are described by Attal, “Ask Dow,” It's All About Paper, Vol. 2, Issue 1, April 2001, Page 8.